CARDIOVASCULAR-RESPONSE TO EXOGENOUS SEROTONIN IN HEALTHY CALVES

Objective-To characterize the cardiovascular response to IV administra tion of serotonin (5-hydroxytryptamine [5-HT]) in carves. Animals-5 he althy unsedated Friesian carves. Procedure-41 5-HT administrations wer e performed: 11 slow infusions (duration, 5 minutes) and 30 bolus infu sions (duration, 5 seconds). Cardiovascular function values were recor ded before, during, and after the infusions. Results-Slow infusion of 5-HT first resulted in a brief period of severe bradycardia, then in s ustained tachycardia with a concomitant increase in cardiac output. Sy stemic blood pressure response to 5-HT was triphasic, with initial hyp otension concomitant with bradycardia, then a presser phase associated with an increase in systemic vascular resistance, and finally, a long -lasting hypotensive phase associated with decreased systemic vascular resistance. Pulmonary hypertension was associated with increased pulm onary vascular resistance, reflecting intense pulmonary vasoconstricti on. Bolus infusion at increasing dosages resulted in dose-dependent br adycardia and systemic hypotension, followed by dose-dependent systemi c hypertension. Unlike with slow infusion, neither the second tachycar dic nor the third systemic hypotensive phases were evident. Conclusion s-5-HT induces dose-dependent cardiovascular responses, including a re flex response followed by pulmonary and systemic vasoconstriction, in healthy calves. Clinical Relevance-Determining the type of serotonergi c receptors responsible for these responses may help to determine whet her 5-HT is involved in the mechanisms underlying brisket disease in c attle.